Download Hot-wire chemical vapour deposition (HWCVD) is a promising

Fabrication, Characterisation and Modelling
of Nanocrystalline Silicon Thin-Film Transistors Obtained by
Hot-Wire Chemical Vapour Deposition
Summary of the thesis presented for the qualification of PhD by
Dosi Dosev
Directors:
Dr. Josep Pallarès
Dr. Joaquim Puigdollers
Tutor: Dr. Ramón Alcubilla
Hot-wire chemical vapour deposition (HWCVD) is a promising technique that permits
polycrystalline silicon films with grain size of nanometers to be obtained at high deposition
rates and low substrate temperatures. This material is expected to have better electronic
properties than the commonly used amorphous hydrogenated silicon (a-Si:H).
In this thesis, thin-film transistors (TFTs) were fabricated using nanocrystalline
hydrogenated silicon film (nc-Si:H), deposited by HWCVD over thermally oxidized silicon
wafer. The employed substrate temperature during the deposition process permits inexpensive
materials as glasses or plastics to be used for various applications in large-area electronics.
Chromium layer was evaporated over the nc-Si:H in order to form drain and source contacts.
Using photolithography techniques, two types of samples were fabricated. The first (simplified)
was with the chromium contacts directly deposited over the intrinsic nc-Si:H layer. Doped n+
layer was incorporated at the drain and source contacts in the second type of samples (complete
samples). The drain and source contacts were electrically separated employing dry etching to
remove the n+ doped layer between them. Dry etching was also employed to eliminate the ncSi:H between the TFTs and to isolate them electrically from each other. The incorporated n+
contact layer improved the performance of the TFTs.
The nc-Si:H TFTs were submitted under prolonged positive and negative gate bias
stress in order to study their stability. The threshold voltage increased under positive gate bias
stress and decreased under negative gate bias stress. After both positive and negative stresses,
the threshold voltage recovered its initial values without annealing. This behaviour indicated
that temporary charge trapping in the channel/gate insulator interface is the responsible process
for the device performance under stress. Measurements of space-charge limited current
confirmed that bulk states were not affected by the positive nor by negative stress.
Analysis of the activation energy and the density of states gave more detailed
information about the physical processes taking place during the stress. Typical drawback of the
nc-Si:H films grown by HWCVD with tungsten (W) filament is the bad quality of the bottom,
initially grown, interfacial layer. It is normally amorphous and porous. We assume that this
property of the nc-Si:H film is determining for charge trapping and the consecutive temporary
changes of the TFT’s characteristics. On the other hand, the absence of defect-state creation
during the gate bias stress demonstrates that the nc-Si:H films did not suffer degradation under
the applied stress conditions.
The electrical characteristics and the operational regimes of the nc-Si:H TFTs were
studied in details in order to obtain the best possible fit using the Spice models for a-Si:H and
poly-Si TFTs existing until now. The analysis of the transconductance gm showed behaviour
typical for a-Si:H TFTs at low gate voltages. In contrast, at high gate voltages unexpected
increasing of gm was observed, as in poly-Si TFTs. Therefore, it was impossible to fit the
transfer and output characteristics with the a-Si:H TFT model neither with poly-Si TFT model.
Numerical simulations performed by means of Silvaco’s Atlas showed that the reason
for this behaviour is the density of acceptor-like states, which situates the properties of nc-Si:H
TFTs between the amorphous and the polycrystalline transistors. Analysis of the concentrations
of the free and the trapped carriers in nc-Si:H layer showed that nc-Si:H operates in transitional
regime between above-threshold and crystalline-like regimes. This transitional regime was
predicted earlier, but not experimentally observed until now. Finally, we introduced new
equations and three new parameters into the existing a-Si TFTs model in order to account for
the transitional regime. The new proposed model permits the shapes of the transconductance,
the transfer and the output characteristics to be modelled accurately.
Doctorand:
/ Dosi Dosev /